As described in patent documents 1 through 3, a seal ring for sealing hydraulic oil is used in a hydraulically driven variable valve timing system (VTC: Valve Timing Control) of an internal combustion engine. For example, as shown in FIGS. 1 through 3, in the variable valve timing system of the patent document 1, a pair of hydraulic oil chambers is formed between the housing (outer rotational body) disposed at the crankshaft side and the vane member (inner rotational body) disposed at the camshaft side. The hydraulic oil chambers are partitioned from each other by the seal ring which seals the annular passages, linked respectively with the hydraulic oil chambers, which are formed between the columnar shaft body and the vane member both formed at the side of the cover. By controlling the oil pressures of the hydraulic oil chambers, the housing and the vane member are rotated relative to each other. Thereby it is possible to transmit the driving force transmitted to the housing from the crankshaft to the camshaft (vane member) by freely changing the rotation phase of the camshaft.
In the seal ring for use in the variable valve timing system, the members to be sealed rotate relative to each other with the axes thereof being greatly misaligned with each other. Thus the clearance between the members to be sealed is set larger than the misalignment amount. Consequently the thickness of the seal ring mounted on the above-described portion is made inevitably large (2.5 to 3.5 mm) in its radial direction. Because the thickness of the seal ring in its radial direction is large, polytetrafluoroethylene (hereinafter referred to as PTFE) resin or the like is hitherto used as the material for use in the variable valve timing system because the polytetrafluoroethylene resin is resistant to breakage when the seal ring is expanded at a seal ring mounting time.